feat: prosediff — word-level prose diff over markdown block structure

The Phase 0 de-risk gate. Segments a document into blocks with goldmark
(headings, paragraphs, list items, code fences, table rows, block quotes,
frontmatter), aligns the two block sequences with Myers over content
hashes, and diffs word-by-word inside modified prose blocks and
line-by-line inside modified code fences.

Whitespace and line wrapping alone produce no diff in prose, and always
do in code — that split is the whole point. Moves are detected by
verbatim anchor and grown over their neighbours, so a relocated section
does not explode into add+remove; a move that also bridges one edited
block is recognised, a section rewritten while moving is not, and that
limit is pinned by a test rather than papered over.

SPIKE.md reports the verdict against twelve real revisions of
docs/DESIGN.md: rewrapping the whole 1139-line document produces 1563
changed lines for git and zero changes here; 77% of real prose
modifications read as small edits; 13% shred and want a two-column
fallback in the web layer, which BlockChange.Similarity already gates.
Verdict: the approach works, build the review UI on it.